2 % (c) The University of Glasgow 2006
3 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
6 TcInstDecls: Typechecking instance declarations
9 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
24 import RnEnv ( lookupGlobalOccRn )
25 import RnSource ( addTcgDUs )
35 import CoreUnfold ( mkDFunUnfolding )
36 -- import CoreUtils ( mkPiTypes )
37 import PrelNames ( inlineIdName )
55 #include "HsVersions.h"
58 Typechecking instance declarations is done in two passes. The first
59 pass, made by @tcInstDecls1@, collects information to be used in the
62 This pre-processed info includes the as-yet-unprocessed bindings
63 inside the instance declaration. These are type-checked in the second
64 pass, when the class-instance envs and GVE contain all the info from
65 all the instance and value decls. Indeed that's the reason we need
66 two passes over the instance decls.
69 Note [How instance declarations are translated]
70 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
71 Here is how we translation instance declarations into Core
75 op1, op2 :: Ix b => a -> b -> b
79 {-# INLINE [2] op1 #-}
83 op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b
87 -- Default methods get the 'self' dictionary as argument
88 -- so they can call other methods at the same type
89 -- Default methods get the same type as their method selector
90 $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b
91 $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). <dm-rhs>
92 -- NB: type variables 'a' and 'b' are *both* in scope in <dm-rhs>
93 -- Note [Tricky type variable scoping]
95 -- A top-level definition for each instance method
96 -- Here op1_i, op2_i are the "instance method Ids"
97 -- The INLINE pragma comes from the user pragma
98 {-# INLINE [2] op1_i #-} -- From the instance decl bindings
99 op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b
100 op1_i = /\a. \(d:C a).
103 -- Note [Subtle interaction of recursion and overlap]
105 local_op1 :: forall b. Ix b => [a] -> b -> b
107 -- Source code; run the type checker on this
108 -- NB: Type variable 'a' (but not 'b') is in scope in <rhs>
109 -- Note [Tricky type variable scoping]
113 op2_i = /\a \d:C a. $dmop2 [a] (df_i a d)
115 -- The dictionary function itself
116 {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions
117 df_i :: forall a. C a -> C [a]
118 df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d)
119 -- But see Note [Default methods in instances]
120 -- We can't apply the type checker to the default-method call
122 -- Use a RULE to short-circuit applications of the class ops
123 {-# RULE "op1@C[a]" forall a, d:C a.
124 op1 [a] (df_i d) = op1_i a d #-}
126 Note [Instances and loop breakers]
127 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
128 * Note that df_i may be mutually recursive with both op1_i and op2_i.
129 It's crucial that df_i is not chosen as the loop breaker, even
130 though op1_i has a (user-specified) INLINE pragma.
132 * Instead the idea is to inline df_i into op1_i, which may then select
133 methods from the MkC record, and thereby break the recursion with
134 df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at
135 the same type, it won't mention df_i, so there won't be recursion in
138 * If op1_i is marked INLINE by the user there's a danger that we won't
139 inline df_i in it, and that in turn means that (since it'll be a
140 loop-breaker because df_i isn't), op1_i will ironically never be
141 inlined. But this is OK: the recursion breaking happens by way of
142 a RULE (the magic ClassOp rule above), and RULES work inside InlineRule
143 unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils
145 Note [ClassOp/DFun selection]
146 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
147 One thing we see a lot is stuff like
149 where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both*
150 'op2' and 'df' to get
151 case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of
152 MkD _ op2 _ _ _ -> op2
153 And that will reduce to ($cop2 d1 d2) which is what we wanted.
155 But it's tricky to make this work in practice, because it requires us to
156 inline both 'op2' and 'df'. But neither is keen to inline without having
157 seen the other's result; and it's very easy to get code bloat (from the
158 big intermediate) if you inline a bit too much.
160 Instead we use a cunning trick.
161 * We arrange that 'df' and 'op2' NEVER inline.
163 * We arrange that 'df' is ALWAYS defined in the sylised form
164 df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ...
166 * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..])
167 that lists its methods.
169 * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return
170 a suitable constructor application -- inlining df "on the fly" as it
173 * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece
174 iff its argument satisfies exprIsConApp_maybe. This is done in
177 * We make 'df' CONLIKE, so that shared uses stil match; eg
179 in ...(op2 d)...(op1 d)...
181 Note [Single-method classes]
182 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
183 If the class has just one method (or, more accurately, just one element
184 of {superclasses + methods}), then we still use the *same* strategy
186 class C a where op :: a -> a
187 instance C a => C [a] where op = <blah>
189 We translate the class decl into a newtype, which just gives
192 axiom Co:C a :: C a ~ (a->a)
194 op :: forall a. C a -> (a -> a)
195 op a d = d |> (Co:C a)
197 MkC :: forall a. (a->a) -> C a
198 MkC = /\a.\op. op |> (sym Co:C a)
200 df :: forall a. C a => C [a]
201 {-# NOINLINE df DFun[ $cop_list ] #-}
202 df = /\a. \d. MkD ($cop_list a d)
204 $cop_list :: forall a. C a => a -> a
207 The "constructor" MkD expands to a cast, as does the class-op selector.
208 The RULE works just like for multi-field dictionaries:
209 * (df a d) returns (Just (MkD,..,[$cop_list a d]))
210 to exprIsConApp_Maybe
212 * The RULE for op picks the right result
214 This is a bit of a hack, because (df a d) isn't *really* a constructor
215 application. But it works just fine in this case, exprIsConApp_maybe
216 is otherwise used only when we hit a case expression which will have
217 a real data constructor in it.
219 The biggest reason for doing it this way, apart form uniformity, is
220 that we want to be very careful when we have
221 instance C a => C [a] where
224 then we'll get an INLINE pragma on $cop_list. The danger is that
225 we'll get something like
226 foo = /\a.\d. $cop_list a d
227 and then we'll eta expand, and then we'll inline TOO EARLY. This happened in
228 Trac #3772 and I spent far too long fiddling arond trying to fix it.
229 Look at the test for Trac #3772.
231 Note [Subtle interaction of recursion and overlap]
232 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
234 class C a where { op1,op2 :: a -> a }
235 instance C a => C [a] where
236 op1 x = op2 x ++ op2 x
238 intance C [Int] where
241 When type-checking the C [a] instance, we need a C [a] dictionary (for
242 the call of op2). If we look up in the instance environment, we find
243 an overlap. And in *general* the right thing is to complain (see Note
244 [Overlapping instances] in InstEnv). But in *this* case it's wrong to
245 complain, because we just want to delegate to the op2 of this same
248 Why is this justified? Because we generate a (C [a]) constraint in
249 a context in which 'a' cannot be instantiated to anything that matches
250 other overlapping instances, or else we would not be excecuting this
251 version of op1 in the first place.
253 It might even be a bit disguised:
255 nullFail :: C [a] => [a] -> [a]
256 nullFail x = op2 x ++ op2 x
258 instance C a => C [a] where
261 Precisely this is used in package 'regex-base', module Context.hs.
262 See the overlapping instances for RegexContext, and the fact that they
263 call 'nullFail' just like the example above. The DoCon package also
264 does the same thing; it shows up in module Fraction.hs
266 Conclusion: when typechecking the methods in a C [a] instance, we want
267 to have C [a] available. That is why we have the strange local
268 definition for 'this' in the definition of op1_i in the example above.
269 We can typecheck the defintion of local_op1, and when doing tcSimplifyCheck
270 we supply 'this' as a given dictionary. Only needed, though, if there
271 are some type variables involved; otherwise there can be no overlap and
274 Note [Tricky type variable scoping]
275 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
278 op1, op2 :: Ix b => a -> b -> b
281 instance C a => C [a]
282 {-# INLINE [2] op1 #-}
285 note that 'a' and 'b' are *both* in scope in <dm-rhs>, but only 'a' is
286 in scope in <rhs>. In particular, we must make sure that 'b' is in
287 scope when typechecking <dm-rhs>. This is achieved by subFunTys,
288 which brings appropriate tyvars into scope. This happens for both
289 <dm-rhs> and for <rhs>, but that doesn't matter: the *renamer* will have
290 complained if 'b' is mentioned in <rhs>.
294 %************************************************************************
296 \subsection{Extracting instance decls}
298 %************************************************************************
300 Gather up the instance declarations from their various sources
303 tcInstDecls1 -- Deal with both source-code and imported instance decls
304 :: [LTyClDecl Name] -- For deriving stuff
305 -> [LInstDecl Name] -- Source code instance decls
306 -> [LDerivDecl Name] -- Source code stand-alone deriving decls
307 -> TcM (TcGblEnv, -- The full inst env
308 [InstInfo Name], -- Source-code instance decls to process;
309 -- contains all dfuns for this module
310 HsValBinds Name) -- Supporting bindings for derived instances
312 tcInstDecls1 tycl_decls inst_decls deriv_decls
314 do { -- Stop if addInstInfos etc discovers any errors
315 -- (they recover, so that we get more than one error each
318 -- (1) Do class and family instance declarations
319 ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }
320 ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1 inst_decls
321 ; idx_tycons <- mapAndRecoverM tcIdxTyInstDeclTL idxty_decls
324 at_tycons_s) = unzip local_info_tycons
325 ; at_idx_tycons = concat at_tycons_s ++ idx_tycons
326 ; clas_decls = filter (isClassDecl.unLoc) tycl_decls
327 ; implicit_things = concatMap implicitTyThings at_idx_tycons
328 ; aux_binds = mkAuxBinds at_idx_tycons
331 -- (2) Add the tycons of indexed types and their implicit
332 -- tythings to the global environment
333 ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {
335 -- (3) Instances from generic class declarations
336 ; generic_inst_info <- getGenericInstances clas_decls
338 -- Next, construct the instance environment so far, consisting
340 -- a) local instance decls
341 -- b) generic instances
342 -- c) local family instance decls
343 ; addInsts local_info $
344 addInsts generic_inst_info $
345 addFamInsts at_idx_tycons $ do {
347 -- (4) Compute instances from "deriving" clauses;
348 -- This stuff computes a context for the derived instance
349 -- decl, so it needs to know about all the instances possible
350 -- NB: class instance declarations can contain derivings as
351 -- part of associated data type declarations
352 failIfErrsM -- If the addInsts stuff gave any errors, don't
353 -- try the deriving stuff, becuase that may give
355 ; (deriv_inst_info, deriv_binds, deriv_dus)
356 <- tcDeriving tycl_decls inst_decls deriv_decls
357 ; gbl_env <- addInsts deriv_inst_info getGblEnv
358 ; return ( addTcgDUs gbl_env deriv_dus,
359 generic_inst_info ++ deriv_inst_info ++ local_info,
360 aux_binds `plusHsValBinds` deriv_binds)
363 -- Make sure that toplevel type instance are not for associated types.
364 -- !!!TODO: Need to perform this check for the TyThing of type functions,
366 tcIdxTyInstDeclTL ldecl@(L loc decl) =
367 do { tything <- tcFamInstDecl ldecl
369 when (isAssocFamily tything) $
370 addErr $ assocInClassErr (tcdName decl)
373 isAssocFamily (ATyCon tycon) =
374 case tyConFamInst_maybe tycon of
375 Nothing -> panic "isAssocFamily: no family?!?"
376 Just (fam, _) -> isTyConAssoc fam
377 isAssocFamily _ = panic "isAssocFamily: no tycon?!?"
379 assocInClassErr :: Name -> SDoc
380 assocInClassErr name =
381 ptext (sLit "Associated type") <+> quotes (ppr name) <+>
382 ptext (sLit "must be inside a class instance")
384 addInsts :: [InstInfo Name] -> TcM a -> TcM a
385 addInsts infos thing_inside
386 = tcExtendLocalInstEnv (map iSpec infos) thing_inside
388 addFamInsts :: [TyThing] -> TcM a -> TcM a
389 addFamInsts tycons thing_inside
390 = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
392 mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
393 mkLocalFamInstTyThing tything = pprPanic "TcInstDcls.addFamInsts"
398 tcLocalInstDecl1 :: LInstDecl Name
399 -> TcM (InstInfo Name, [TyThing])
400 -- A source-file instance declaration
401 -- Type-check all the stuff before the "where"
403 -- We check for respectable instance type, and context
404 tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
406 addErrCtxt (instDeclCtxt1 poly_ty) $
408 do { is_boot <- tcIsHsBoot
409 ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
412 ; (tyvars, theta, tau) <- tcHsInstHead poly_ty
414 -- Now, check the validity of the instance.
415 ; (clas, inst_tys) <- checkValidInstHead tau
416 ; checkValidInstance tyvars theta clas inst_tys
418 -- Next, process any associated types.
419 ; idx_tycons <- recoverM (return []) $
420 do { idx_tycons <- checkNoErrs $ mapAndRecoverM tcFamInstDecl ats
421 ; checkValidAndMissingATs clas (tyvars, inst_tys)
423 ; return idx_tycons }
425 -- Finally, construct the Core representation of the instance.
426 -- (This no longer includes the associated types.)
427 ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
428 -- Dfun location is that of instance *header*
429 ; overlap_flag <- getOverlapFlag
430 ; let (eq_theta,dict_theta) = partition isEqPred theta
431 theta' = eq_theta ++ dict_theta
432 dfun = mkDictFunId dfun_name tyvars theta' clas inst_tys
433 ispec = mkLocalInstance dfun overlap_flag
435 ; return (InstInfo { iSpec = ispec,
436 iBinds = VanillaInst binds uprags False },
440 -- We pass in the source form and the type checked form of the ATs. We
441 -- really need the source form only to be able to produce more informative
443 checkValidAndMissingATs :: Class
444 -> ([TyVar], [TcType]) -- instance types
445 -> [(LTyClDecl Name, -- source form of AT
446 TyThing)] -- Core form of AT
448 checkValidAndMissingATs clas inst_tys ats
449 = do { -- Issue a warning for each class AT that is not defined in this
451 ; let class_ats = map tyConName (classATs clas)
452 defined_ats = listToNameSet . map (tcdName.unLoc.fst) $ ats
453 omitted = filterOut (`elemNameSet` defined_ats) class_ats
454 ; warn <- doptM Opt_WarnMissingMethods
455 ; mapM_ (warnTc warn . omittedATWarn) omitted
457 -- Ensure that all AT indexes that correspond to class parameters
458 -- coincide with the types in the instance head. All remaining
459 -- AT arguments must be variables. Also raise an error for any
460 -- type instances that are not associated with this class.
461 ; mapM_ (checkIndexes clas inst_tys) ats
464 checkIndexes clas inst_tys (hsAT, ATyCon tycon)
465 -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
466 = checkIndexes' clas inst_tys hsAT
468 snd . fromJust . tyConFamInst_maybe $ tycon)
469 checkIndexes _ _ _ = panic "checkIndexes"
471 checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
472 = let atName = tcdName . unLoc $ hsAT
474 setSrcSpan (getLoc hsAT) $
475 addErrCtxt (atInstCtxt atName) $
476 case find ((atName ==) . tyConName) (classATs clas) of
477 Nothing -> addErrTc $ badATErr clas atName -- not in this class
479 case assocTyConArgPoss_maybe atycon of
480 Nothing -> panic "checkIndexes': AT has no args poss?!?"
483 -- The following is tricky! We need to deal with three
484 -- complications: (1) The AT possibly only uses a subset of
485 -- the class parameters as indexes and those it uses may be in
486 -- a different order; (2) the AT may have extra arguments,
487 -- which must be type variables; and (3) variables in AT and
488 -- instance head will be different `Name's even if their
489 -- source lexemes are identical.
491 -- e.g. class C a b c where
492 -- data D b a :: * -> * -- NB (1) b a, omits c
493 -- instance C [x] Bool Char where
494 -- data D Bool [x] v = MkD x [v] -- NB (2) v
495 -- -- NB (3) the x in 'instance C...' have differnt
496 -- -- Names to x's in 'data D...'
498 -- Re (1), `poss' contains a permutation vector to extract the
499 -- class parameters in the right order.
501 -- Re (2), we wrap the (permuted) class parameters in a Maybe
502 -- type and use Nothing for any extra AT arguments. (First
503 -- equation of `checkIndex' below.)
505 -- Re (3), we replace any type variable in the AT parameters
506 -- that has the same source lexeme as some variable in the
507 -- instance types with the instance type variable sharing its
510 let relevantInstTys = map (instTys !!) poss
511 instArgs = map Just relevantInstTys ++
512 repeat Nothing -- extra arguments
513 renaming = substSameTyVar atTvs instTvs
515 zipWithM_ checkIndex (substTys renaming atTys) instArgs
517 checkIndex ty Nothing
518 | isTyVarTy ty = return ()
519 | otherwise = addErrTc $ mustBeVarArgErr ty
520 checkIndex ty (Just instTy)
521 | ty `tcEqType` instTy = return ()
522 | otherwise = addErrTc $ wrongATArgErr ty instTy
524 listToNameSet = addListToNameSet emptyNameSet
526 substSameTyVar [] _ = emptyTvSubst
527 substSameTyVar (tv:tvs) replacingTvs =
528 let replacement = case find (tv `sameLexeme`) replacingTvs of
529 Nothing -> mkTyVarTy tv
530 Just rtv -> mkTyVarTy rtv
532 tv1 `sameLexeme` tv2 =
533 nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
535 extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement
539 %************************************************************************
541 Type-checking instance declarations, pass 2
543 %************************************************************************
546 tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
547 -> TcM (LHsBinds Id, TcLclEnv)
548 -- (a) From each class declaration,
549 -- generate any default-method bindings
550 -- (b) From each instance decl
551 -- generate the dfun binding
553 tcInstDecls2 tycl_decls inst_decls
554 = do { -- (a) Default methods from class decls
555 let class_decls = filter (isClassDecl . unLoc) tycl_decls
556 ; (dm_ids_s, dm_binds_s) <- mapAndUnzipM tcClassDecl2 class_decls
558 ; tcExtendIdEnv (concat dm_ids_s) $ do
560 -- (b) instance declarations
561 { inst_binds_s <- mapM tcInstDecl2 inst_decls
564 ; let binds = unionManyBags dm_binds_s `unionBags`
565 unionManyBags inst_binds_s
566 ; tcl_env <- getLclEnv -- Default method Ids in here
567 ; return (binds, tcl_env) } }
569 tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
570 tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
571 = recoverM (return emptyLHsBinds) $
573 addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
574 tc_inst_decl2 dfun_id ibinds
576 dfun_id = instanceDFunId ispec
577 loc = getSrcSpan dfun_id
582 tc_inst_decl2 :: Id -> InstBindings Name -> TcM (LHsBinds Id)
583 -- Returns a binding for the dfun
585 ------------------------
586 -- Derived newtype instances; surprisingly tricky!
588 -- class Show a => Foo a b where ...
589 -- newtype N a = MkN (Tree [a]) deriving( Foo Int )
591 -- The newtype gives an FC axiom looking like
592 -- axiom CoN a :: N a ~ Tree [a]
593 -- (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
595 -- So all need is to generate a binding looking like:
596 -- dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
597 -- dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
598 -- case df `cast` (Foo Int (sym (CoN a))) of
599 -- Foo _ op1 .. opn -> Foo ds op1 .. opn
601 -- If there are no superclasses, matters are simpler, because we don't need the case
602 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
604 tc_inst_decl2 dfun_id (NewTypeDerived coi)
605 = do { let rigid_info = InstSkol
606 origin = SigOrigin rigid_info
607 inst_ty = idType dfun_id
608 inst_tvs = fst (tcSplitForAllTys inst_ty)
609 ; (inst_tvs', theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
610 -- inst_head_ty is a PredType
612 ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
613 (class_tyvars, sc_theta, _, _) = classBigSig cls
614 cls_tycon = classTyCon cls
615 sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
616 Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
620 IdCo -> (last_ty, idHsWrapper)
621 ACo co -> (snd (coercionKind co'), WpCast (mk_full_coercion co'))
623 co' = substTyWith inst_tvs (mkTyVarTys inst_tvs') co
624 -- NB: the free variable of coi are bound by the
625 -- universally quantified variables of the dfun_id
626 -- This is weird, and maybe we should make NewTypeDerived
627 -- carry a type-variable list too; but it works fine
629 -----------------------
631 -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
632 -- But we want the coercion (C s1 .. sm (sym (CoT a1 .. ak)))
633 -- with kind (C s1 .. sm (T a1 .. ak) ~ C s1 .. sm <rep_ty>)
634 -- where rep_ty is the (eta-reduced) type rep of T
635 -- So we just replace T with CoT, and insert a 'sym'
636 -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
638 mk_full_coercion co = mkTyConApp cls_tycon
639 (initial_cls_inst_tys ++ [mkSymCoercion co])
640 -- Full coercion : (Foo Int (Tree [a]) ~ Foo Int (N a)
642 rep_pred = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
643 -- In our example, rep_pred is (Foo Int (Tree [a]))
645 ; sc_loc <- getInstLoc InstScOrigin
646 ; sc_dicts <- newDictBndrs sc_loc sc_theta'
647 ; inst_loc <- getInstLoc origin
648 ; dfun_dicts <- newDictBndrs inst_loc theta
649 ; rep_dict <- newDictBndr inst_loc rep_pred
650 ; this_dict <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
652 -- Figure out bindings for the superclass context from dfun_dicts
653 -- Don't include this_dict in the 'givens', else
654 -- sc_dicts get bound by just selecting from this_dict!!
655 ; sc_binds <- addErrCtxt superClassCtxt $
656 tcSimplifySuperClasses inst_loc this_dict dfun_dicts
659 -- It's possible that the superclass stuff might unified something
660 -- in the envt with one of the clas_tyvars
661 ; checkSigTyVars inst_tvs'
663 ; let coerced_rep_dict = wrapId wrapper (instToId rep_dict)
665 ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
666 ; let dict_bind = mkVarBind (instToId this_dict) (noLoc body)
668 ; return (unitBag $ noLoc $
669 AbsBinds inst_tvs' (map instToVar dfun_dicts)
670 [(inst_tvs', dfun_id, instToId this_dict, [])]
671 (dict_bind `consBag` sc_binds)) }
673 -----------------------
674 -- (make_body C tys scs coreced_rep_dict)
676 -- (case coerced_rep_dict of { C _ ops -> C scs ops })
677 -- But if there are no superclasses, it returns just coerced_rep_dict
678 -- See Note [Newtype deriving superclasses] in TcDeriv.lhs
680 make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
681 | null sc_dicts -- Case (a)
682 = return coerced_rep_dict
683 | otherwise -- Case (b)
684 = do { op_ids <- newSysLocalIds (fsLit "op") op_tys
685 ; dummy_sc_dict_ids <- newSysLocalIds (fsLit "sc") (map idType sc_dict_ids)
686 ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
687 pat_dicts = dummy_sc_dict_ids,
688 pat_binds = emptyLHsBinds,
689 pat_args = PrefixCon (map nlVarPat op_ids),
691 the_match = mkSimpleMatch [noLoc the_pat] the_rhs
692 the_rhs = mkHsConApp cls_data_con cls_inst_tys $
693 map HsVar (sc_dict_ids ++ op_ids)
695 -- Warning: this HsCase scrutinises a value with a PredTy, which is
696 -- never otherwise seen in Haskell source code. It'd be
697 -- nicer to generate Core directly!
698 ; return (HsCase (noLoc coerced_rep_dict) $
699 MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
701 sc_dict_ids = map instToId sc_dicts
702 pat_ty = mkTyConApp cls_tycon cls_inst_tys
703 cls_data_con = head (tyConDataCons cls_tycon)
704 cls_arg_tys = dataConInstArgTys cls_data_con cls_inst_tys
705 op_tys = dropList sc_dict_ids cls_arg_tys
707 ------------------------
708 -- Ordinary instances
710 tc_inst_decl2 dfun_id (VanillaInst monobinds uprags standalone_deriv)
711 = do { let rigid_info = InstSkol
712 inst_ty = idType dfun_id
713 loc = getSrcSpan dfun_id
715 -- Instantiate the instance decl with skolem constants
716 ; (inst_tyvars', dfun_theta', inst_head') <- tcSkolSigType rigid_info inst_ty
717 -- These inst_tyvars' scope over the 'where' part
718 -- Those tyvars are inside the dfun_id's type, which is a bit
719 -- bizarre, but OK so long as you realise it!
721 (clas, inst_tys') = tcSplitDFunHead inst_head'
722 (class_tyvars, sc_theta, sc_sels, op_items) = classBigSig clas
724 -- Instantiate the super-class context with inst_tys
725 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
726 origin = SigOrigin rigid_info
728 -- Create dictionary Ids from the specified instance contexts.
729 ; inst_loc <- getInstLoc origin
730 ; dfun_dicts <- newDictBndrs inst_loc dfun_theta' -- Includes equalities
731 ; this_dict <- newDictBndr inst_loc (mkClassPred clas inst_tys')
732 -- Default-method Ids may be mentioned in synthesised RHSs,
733 -- but they'll already be in the environment.
736 -- Cook up a binding for "this = df d1 .. dn",
737 -- to use in each method binding
738 -- Need to clone the dict in case it is floated out, and
739 -- then clashes with its friends
740 ; cloned_this <- cloneDict this_dict
741 ; let cloned_this_bind = mkVarBind (instToId cloned_this) $
742 L loc $ wrapId app_wrapper dfun_id
743 app_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
744 dfun_lam_vars = map instToVar dfun_dicts -- Includes equalities
746 | null inst_tyvars' && null dfun_theta' = (this_dict, emptyBag)
747 | otherwise = (cloned_this, unitBag cloned_this_bind)
749 -- Deal with 'SPECIALISE instance' pragmas
750 -- See Note [SPECIALISE instance pragmas]
751 ; let spec_inst_sigs = filter isSpecInstLSig uprags
752 -- The filter removes the pragmas for methods
753 ; spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) spec_inst_sigs
755 -- Typecheck the methods
756 ; let prag_fn = mkPragFun uprags
757 tc_meth = tcInstanceMethod loc standalone_deriv
761 prag_fn spec_inst_prags monobinds
763 ; (meth_ids, meth_binds) <- tcExtendTyVarEnv inst_tyvars' $
764 mapAndUnzipM tc_meth op_items
766 -- Figure out bindings for the superclass context
767 ; sc_loc <- getInstLoc InstScOrigin
768 ; sc_dicts <- newDictOccs sc_loc sc_theta' -- These are wanted
769 ; let tc_sc = tcSuperClass inst_loc inst_tyvars' dfun_dicts nested_this_pair
770 ; (sc_ids, sc_binds) <- mapAndUnzipM tc_sc (sc_sels `zip` sc_dicts)
772 -- It's possible that the superclass stuff might unified
773 -- something in the envt with one of the inst_tyvars'
774 ; checkSigTyVars inst_tyvars'
776 -- Create the result bindings
777 ; let dict_constr = classDataCon clas
778 this_dict_id = instToId this_dict
779 dict_bind = mkVarBind this_dict_id dict_rhs
780 dict_rhs = foldl mk_app inst_constr (sc_ids ++ meth_ids)
781 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
782 (dataConWrapId dict_constr)
783 -- We don't produce a binding for the dict_constr; instead we
784 -- rely on the simplifier to unfold this saturated application
785 -- We do this rather than generate an HsCon directly, because
786 -- it means that the special cases (e.g. dictionary with only one
787 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
788 -- than needing to be repeated here.
790 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
791 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
792 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
794 -- Do not inline the dfun; instead give it a magic DFunFunfolding
795 -- See Note [ClassOp/DFun selection]
796 -- See also note [Single-method classes]
797 dfun_id_w_fun = dfun_id
798 `setIdUnfolding` mkDFunUnfolding dict_constr (sc_ids ++ meth_ids)
799 `setInlinePragma` dfunInlinePragma
804 [(inst_tyvars', dfun_id_w_fun, this_dict_id, spec_inst_prags)]
807 ; showLIE (text "instance")
808 ; return (unitBag (L loc main_bind) `unionBags`
809 listToBag meth_binds `unionBags`
814 -- Create the result bindings
815 ; let this_dict_id = instToId this_dict
816 arg_ids = sc_ids ++ meth_ids
817 arg_binds = listToBag meth_binds `unionBags`
820 ; showLIE (text "instance")
821 ; case newTyConCo_maybe (classTyCon clas) of
822 Nothing -- A multi-method class
823 -> return (unitBag (L loc data_bind) `unionBags` arg_binds)
825 data_dfun_id = dfun_id -- Do not inline; instead give it a magic DFunFunfolding
826 -- See Note [ClassOp/DFun selection]
827 `setIdUnfolding` mkDFunUnfolding dict_constr arg_ids
828 `setInlinePragma` dfunInlinePragma
830 data_bind = AbsBinds inst_tyvars' dfun_lam_vars
831 [(inst_tyvars', data_dfun_id, this_dict_id, spec_inst_prags)]
834 dict_bind = mkVarBind this_dict_id dict_rhs
835 dict_rhs = foldl mk_app inst_constr arg_ids
836 dict_constr = classDataCon clas
837 inst_constr = L loc $ wrapId (mkWpTyApps inst_tys')
838 (dataConWrapId dict_constr)
839 -- We don't produce a binding for the dict_constr; instead we
840 -- rely on the simplifier to unfold this saturated application
841 -- We do this rather than generate an HsCon directly, because
842 -- it means that the special cases (e.g. dictionary with only one
843 -- member) are dealt with by the common MkId.mkDataConWrapId code rather
844 -- than needing to be repeated here.
846 mk_app :: LHsExpr Id -> Id -> LHsExpr Id
847 mk_app fun arg_id = L loc (HsApp fun (L loc (wrapId arg_wrapper arg_id)))
848 arg_wrapper = mkWpApps dfun_lam_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars')
850 Just the_nt_co -- (Just co) for a single-method class
851 -> return (unitBag (L loc nt_bind) `unionBags` arg_binds)
853 nt_dfun_id = dfun_id -- Just let the dfun inline; see Note [Single-method classes]
854 `setInlinePragma` alwaysInlinePragma
856 local_nt_dfun = setIdType this_dict_id inst_ty -- A bit of a hack, but convenient
858 nt_bind = AbsBinds [] []
859 [([], nt_dfun_id, local_nt_dfun, spec_inst_prags)]
860 (unitBag (mkVarBind local_nt_dfun (L loc (wrapId nt_cast the_meth_id))))
862 the_meth_id = ASSERT( length arg_ids == 1 ) head arg_ids
863 nt_cast = WpCast $ mkPiTypes (inst_tyvars' ++ dfun_lam_vars) $
864 mkSymCoercion (mkTyConApp the_nt_co inst_tys')
867 ------------------------------
868 tcSuperClass :: InstLoc -> [TyVar] -> [Inst]
869 -> (Inst, LHsBinds Id)
870 -> (Id, Inst) -> TcM (Id, LHsBind Id)
871 -- Build a top level decl like
872 -- sc_op = /\a \d. let this = ... in
875 -- The "this" part is just-in-case (discarded if not used)
876 -- See Note [Recursive superclasses]
877 tcSuperClass inst_loc tyvars dicts (this_dict, this_bind)
879 = addErrCtxt superClassCtxt $
880 do { sc_binds <- tcSimplifySuperClasses inst_loc
881 this_dict dicts [sc_dict]
882 -- Don't include this_dict in the 'givens', else
883 -- sc_dicts get bound by just selecting from this_dict!!
886 ; let sc_op_ty = mkSigmaTy tyvars (map dictPred dicts)
887 (mkPredTy (dictPred sc_dict))
888 sc_op_name = mkDerivedInternalName mkClassOpAuxOcc uniq
890 sc_op_id = mkLocalId sc_op_name sc_op_ty
891 sc_id = instToVar sc_dict
892 sc_op_bind = AbsBinds tyvars
893 (map instToVar dicts)
894 [(tyvars, sc_op_id, sc_id, [])]
895 (this_bind `unionBags` sc_binds)
897 ; return (sc_op_id, noLoc sc_op_bind) }
900 Note [Recursive superclasses]
901 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
902 See Trac #1470 for why we would *like* to add "this_dict" to the
903 available instances here. But we can't do so because then the superclases
904 get satisfied by selection from this_dict, and that leads to an immediate
905 loop. What we need is to add this_dict to Avails without adding its
906 superclasses, and we currently have no way to do that.
908 Note [SPECIALISE instance pragmas]
909 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
912 instance (Ix a, Ix b) => Ix (a,b) where
913 {-# SPECIALISE instance Ix (Int,Int) #-}
916 We do *not* want to make a specialised version of the dictionary
917 function. Rather, we want specialised versions of each method.
918 Thus we should generate something like this:
920 $dfIx :: (Ix a, Ix x) => Ix (a,b)
921 {- DFUN [$crange, ...] -}
922 $dfIx da db = Ix ($crange da db) (...other methods...)
924 $dfIxPair :: (Ix a, Ix x) => Ix (a,b)
925 {- DFUN [$crangePair, ...] -}
926 $dfIxPair = Ix ($crangePair da db) (...other methods...)
928 $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)]
929 {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-}
930 $crange da db = <blah>
932 {-# RULE range ($dfIx da db) = $crange da db #-}
936 * The RULE is unaffected by the specialisation. We don't want to
937 specialise $dfIx, because then it would need a specialised RULE
938 which is a pain. The single RULE works fine at all specialisations.
939 See Note [How instance declarations are translated] above
941 * Instead, we want to specialise the *method*, $crange
943 In practice, rather than faking up a SPECIALISE pragama for each
944 method (which is painful, since we'd have to figure out its
945 specialised type), we call tcSpecPrag *as if* were going to specialise
946 $dfIx -- you can see that in the call to tcSpecInst. That generates a
947 SpecPrag which, as it turns out, can be used unchanged for each method.
948 The "it turns out" bit is delicate, but it works fine!
951 tcSpecInst :: Id -> Sig Name -> TcM SpecPrag
952 tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
953 = addErrCtxt (spec_ctxt prag) $
954 do { let name = idName dfun_id
955 ; (tyvars, theta, tau) <- tcHsInstHead hs_ty
956 ; let spec_ty = mkSigmaTy tyvars theta tau
957 ; co_fn <- tcSubExp (SpecPragOrigin name) (idType dfun_id) spec_ty
958 ; return (SpecPrag co_fn defaultInlinePragma) }
960 spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)
962 tcSpecInst _ _ = panic "tcSpecInst"
965 %************************************************************************
967 Type-checking an instance method
969 %************************************************************************
972 - Make the method bindings, as a [(NonRec, HsBinds)], one per method
973 - Remembering to use fresh Name (the instance method Name) as the binder
974 - Bring the instance method Ids into scope, for the benefit of tcInstSig
975 - Use sig_fn mapping instance method Name -> instance tyvars
977 - Use tcValBinds to do the checking
980 tcInstanceMethod :: SrcSpan -> Bool -> Class -> [TcTyVar] -> [Inst]
982 -> (Inst, LHsBinds Id) -- "This" and its binding
983 -> TcPragFun -- Local prags
984 -> [LSpecPrag] -- Arising from 'SPECLALISE instance'
987 -> TcM (Id, LHsBind Id)
988 -- The returned inst_meth_ids all have types starting
989 -- forall tvs. theta => ...
991 tcInstanceMethod loc standalone_deriv clas tyvars dfun_dicts inst_tys
992 (this_dict, this_dict_bind)
993 prag_fn spec_inst_prags binds_in (sel_id, dm_info)
994 = do { uniq <- newUnique
995 ; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
996 ; local_meth_name <- newLocalName sel_name
997 -- Base the local_meth_name on the selector name, becuase
998 -- type errors from tcInstanceMethodBody come from here
1000 ; let local_meth_ty = instantiateMethod clas sel_id inst_tys
1001 meth_ty = mkSigmaTy tyvars (map dictPred dfun_dicts) local_meth_ty
1002 meth_id = mkLocalId meth_name meth_ty
1003 local_meth_id = mkLocalId local_meth_name local_meth_ty
1007 = add_meth_ctxt rn_bind $
1008 do { (meth_id1, spec_prags) <- tcPrags NonRecursive False True
1009 meth_id (prag_fn sel_name)
1010 ; tcInstanceMethodBody (instLoc this_dict)
1012 ([this_dict], this_dict_bind)
1013 meth_id1 local_meth_id
1015 (spec_inst_prags ++ spec_prags)
1019 tc_default :: DefMeth -> TcM (Id, LHsBind Id)
1020 -- The user didn't supply a method binding, so we have to make
1021 -- up a default binding, in a way depending on the default-method info
1023 tc_default NoDefMeth -- No default method at all
1024 = do { warnMissingMethod sel_id
1025 ; return (meth_id, mkVarBind meth_id $
1026 mkLHsWrap lam_wrapper error_rhs) }
1028 tc_default GenDefMeth -- Derivable type classes stuff
1029 = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id local_meth_name
1030 ; tc_body meth_bind }
1032 tc_default DefMeth -- An polymorphic default method
1033 = do { -- Build the typechecked version directly,
1034 -- without calling typecheck_method;
1035 -- see Note [Default methods in instances]
1036 -- Generate /\as.\ds. let this = df as ds
1037 -- in $dm inst_tys this
1038 -- The 'let' is necessary only because HsSyn doesn't allow
1039 -- you to apply a function to a dictionary *expression*.
1040 dm_name <- lookupGlobalOccRn (mkDefMethRdrName sel_name)
1041 -- Might not be imported, but will be an OrigName
1042 ; dm_id <- tcLookupId dm_name
1043 ; inline_id <- tcLookupId inlineIdName
1044 ; let dm_inline_prag = idInlinePragma dm_id
1045 dm_app = HsWrap (WpApp (instToId this_dict) <.> mkWpTyApps inst_tys) $
1047 rhs | isInlinePragma dm_inline_prag -- See Note [INLINE and default methods]
1048 = HsApp (L loc (HsWrap (WpTyApp local_meth_ty) (HsVar inline_id)))
1050 | otherwise = dm_app
1052 meth_bind = L loc $ VarBind { var_id = local_meth_id
1053 , var_rhs = L loc rhs
1054 , var_inline = False }
1055 meth_id1 = meth_id `setInlinePragma` dm_inline_prag
1056 -- Copy the inline pragma (if any) from the default
1057 -- method to this version. Note [INLINE and default methods]
1059 bind = AbsBinds { abs_tvs = tyvars, abs_dicts = dfun_lam_vars
1060 , abs_exports = [( tyvars, meth_id1
1061 , local_meth_id, spec_inst_prags)]
1062 , abs_binds = this_dict_bind `unionBags` unitBag meth_bind }
1063 -- Default methods in an instance declaration can't have their own
1064 -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
1065 -- currently they are rejected with
1066 -- "INLINE pragma lacks an accompanying binding"
1068 ; return (meth_id1, L loc bind) }
1070 ; case findMethodBind sel_name local_meth_name binds_in of
1071 Just user_bind -> tc_body user_bind -- User-supplied method binding
1072 Nothing -> tc_default dm_info -- None supplied
1075 sel_name = idName sel_id
1077 meth_sig_fn _ = Just [] -- The 'Just' says "yes, there's a type sig"
1078 -- But there are no scoped type variables from local_method_id
1079 -- Only the ones from the instance decl itself, which are already
1080 -- in scope. Example:
1081 -- class C a where { op :: forall b. Eq b => ... }
1082 -- instance C [c] where { op = <rhs> }
1083 -- In <rhs>, 'c' is scope but 'b' is not!
1085 error_rhs = L loc $ HsApp error_fun error_msg
1086 error_fun = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
1087 error_msg = L loc (HsLit (HsStringPrim (mkFastString error_string)))
1088 meth_tau = funResultTy (applyTys (idType sel_id) inst_tys)
1089 error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
1091 dfun_lam_vars = map instToVar dfun_dicts
1092 lam_wrapper = mkWpTyLams tyvars <.> mkWpLams dfun_lam_vars
1094 -- For instance decls that come from standalone deriving clauses
1095 -- we want to print out the full source code if there's an error
1096 -- because otherwise the user won't see the code at all
1097 add_meth_ctxt rn_bind thing
1098 | standalone_deriv = addLandmarkErrCtxt (derivBindCtxt clas inst_tys rn_bind) thing
1101 wrapId :: HsWrapper -> id -> HsExpr id
1102 wrapId wrapper id = mkHsWrap wrapper (HsVar id)
1104 derivBindCtxt :: Class -> [Type ] -> LHsBind Name -> SDoc
1105 derivBindCtxt clas tys bind
1106 = vcat [ ptext (sLit "When typechecking a standalone-derived method for")
1107 <+> quotes (pprClassPred clas tys) <> colon
1108 , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]
1110 warnMissingMethod :: Id -> TcM ()
1111 warnMissingMethod sel_id
1112 = do { warn <- doptM Opt_WarnMissingMethods
1113 ; warnTc (warn -- Warn only if -fwarn-missing-methods
1114 && not (startsWithUnderscore (getOccName sel_id)))
1115 -- Don't warn about _foo methods
1116 (ptext (sLit "No explicit method nor default method for")
1117 <+> quotes (ppr sel_id)) }
1120 Note [Export helper functions]
1121 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1122 We arrange to export the "helper functions" of an instance declaration,
1123 so that they are not subject to preInlineUnconditionally, even if their
1124 RHS is trivial. Reason: they are mentioned in the DFunUnfolding of
1125 the dict fun as Ids, not as CoreExprs, so we can't substitute a
1126 non-variable for them.
1128 We could change this by making DFunUnfoldings have CoreExprs, but it
1129 seems a bit simpler this way.
1131 Note [Default methods in instances]
1132 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1139 instance Baz Int Int
1141 From the class decl we get
1143 $dmfoo :: forall v x. Baz v x => x -> x
1146 Notice that the type is ambiguous. That's fine, though. The instance decl generates
1148 $dBazIntInt = MkBaz fooIntInt
1149 fooIntInt = $dmfoo Int Int $dBazIntInt
1151 BUT this does mean we must generate the dictionary translation of
1152 fooIntInt directly, rather than generating source-code and
1153 type-checking it. That was the bug in Trac #1061. In any case it's
1154 less work to generate the translated version!
1156 Note [INLINE and default methods]
1157 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1158 We *copy* any INLINE pragma from the default method to the instance.
1161 op1, op2 :: Bool -> a -> a
1164 op1 b x = op2 (not b) x
1166 instance Foo Int where
1171 {-# INLINE $dmop1 #-}
1172 $dmop1 d b x = op2 d (not b) x
1174 $fFooInt = MkD $cop1 $cop2
1176 {-# INLINE $cop1 #-}
1177 $cop1 = inline $dmop1 $fFooInt
1182 a) We copy $dmop1's inline pragma to $cop1. Otherwise
1183 we'll just inline the former in the latter and stop, which
1184 isn't what the user expected
1186 b) We use the magic 'inline' Id to ensure that $dmop1 really is
1187 inlined in $cop1, even though
1188 (i) the latter itself has an INLINE pragma
1189 (ii) $dmop1 is not saturated
1190 That is important to allow the mutual recursion between $fooInt and
1194 %************************************************************************
1196 \subsection{Error messages}
1198 %************************************************************************
1201 instDeclCtxt1 :: LHsType Name -> SDoc
1202 instDeclCtxt1 hs_inst_ty
1203 = inst_decl_ctxt (case unLoc hs_inst_ty of
1204 HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
1205 HsPredTy pred -> ppr pred
1206 _ -> ppr hs_inst_ty) -- Don't expect this
1207 instDeclCtxt2 :: Type -> SDoc
1208 instDeclCtxt2 dfun_ty
1209 = inst_decl_ctxt (ppr (mkClassPred cls tys))
1211 (_,cls,tys) = tcSplitDFunTy dfun_ty
1213 inst_decl_ctxt :: SDoc -> SDoc
1214 inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc
1216 superClassCtxt :: SDoc
1217 superClassCtxt = ptext (sLit "When checking the super-classes of an instance declaration")
1219 atInstCtxt :: Name -> SDoc
1220 atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
1223 mustBeVarArgErr :: Type -> SDoc
1224 mustBeVarArgErr ty =
1225 sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
1226 ptext (sLit "must be variables")
1227 , ptext (sLit "Instead of a variable, found") <+> ppr ty
1230 wrongATArgErr :: Type -> Type -> SDoc
1231 wrongATArgErr ty instTy =
1232 sep [ ptext (sLit "Type indexes must match class instance head")
1233 , ptext (sLit "Found") <+> quotes (ppr ty)
1234 <+> ptext (sLit "but expected") <+> quotes (ppr instTy)